1. Field of the Invention
The present invention relates to an injection molding method and an injection molding die.
2. Description of the Related Art
In a general injection molding die, a molten resin in a high-temperature state is injected from an injection molding machine, and supplied to a runner via a sprue formed in a fixed die. The molten resin is then supplied to a cavity portion of a die from a distal end portion of the runner through a gate portion. Here, when a molded article is a resin lens, the temperature of the molten resin is set much higher than the glass transition point temperature of the molten resin so that the molten resin has fluidity.
The thickness of the lens in optical axis direction generally varies between a center portion of the lens and an outer circumferential part of the lens. For example, in a concave lens, a center portion of the lens is thinner than an outer circumferential part of the lens. For example, in a convex lens, a center portion of the lens is thicker than an outer circumferential part of the lens. The flow of the molten resin in the cavity portion tends to change depending on the change of the thickness along the optical axis direction.
For example, when the molten resin is in contact with an inner surface of the cavity portion, and the temperature of the inner surface of the cavity portion is lower than the glass transition point temperature of the molten resin, in which case the heat of the molten resin is conducted to the die from one part of the molten resin which is in close contact with the inner surface of the cavity portion. Thus, the fluidity of the side of the molten resin which is in close contact with the inner surface of the cavity portion becomes lower. In contrast, at a position located away from the inner surface of the cavity portion, heat conduction is inhibited by the molten resin existing between another part of the molten resin which is located away and the inner surface of the cavity portion. Thus, the fluidity of the molten resin is maintained. Therefore, in the cavity portion of the injection molding die for the lens in which the thickness in the optical axis direction varies between the center portion of the lens and the outer circumferential part of the lens, a better condition of the fluidity of the molten resin tends to be maintained in the thicker parts of the cavity portion in the optical axis direction than in thinner parts. As a result, the flow velocity of the molten resin running through the thicker parts is higher than the flow velocity of the molten resin running through the thinner parts. The flow velocity of the molten resin is lower when the thickness of the cavity portion (i.e., the distance between the inner surfaces) is smaller, and in an extreme case, there is a fear that the molten resin may stop without flowing. If the molten resin stops in the cavity portion, the molten resin releases heat to the die and immediately cures.
Thus, in the molten resins which have flowed into the cavity portion from the gate portion, a molten resin A flowing in the thin parts of the cavity portion extremely slowly flows or is stopped. In contrast, a molten resin B flowing in the thick parts of the cavity portion flows in such a manner as to avoid the molten resin A. This molten resin B then flows to the side of one part of the cavity portion located opposite to the gate portion around the molten resin A. In this instance, the molten resin B flows back to the void part resulting from the molten resin A that has stopped, and there is a possibility that the void part may be filled with the molten resin B. When the molten resins A and B which are different in flowing direction are associated in the cavity portion, there is a strong possibility that the molten resins A and B which have flowed while being decreased in temperature and have been associated with each other may not be homogeneously unified and weld lines may thus be generated. Between the molten resin A and the molten resin B in particular, the molten resin A may cure to some degree as described above, and weld lines are generated.
For example, in the cavity portion of the concave lens, the center portion of the lens is thinner than the outer circumferential part of the lens. Thus, in the molten resins which have flowed into the cavity portion from the part of the gate portion side, the flow velocity of the molten resin running through the center portion of the lens is lower than the flow velocity of the molten resin running through the outer circumferential part of the lens. This phenomenon becomes more evident from a point near the central line and thereafter in the cavity portion. When an uneven thickness degree M (M=T2/T1) which is the ratio between a thickness T1 of the center portion of the lens and a thickness T2 of the outer circumferential part of the lens is high (e.g., M is 4 or more), in the molten resin which has flowed into the cavity portion from the part of the gate portion side, there occurs a phenomenon in which the head portion (flow front portion) of the molten resin which has passed the right side of the center portion of the lens and the head portion of the molten resin which has passed the left side of the center portion of the lens associate with each other ahead of the center portion of the lens. In this case, air collects in the cavity portion by this association, and parts that are not transferred to the injection molding die and weld lines are generated, thus the appearance of the molding becomes defective.
In the convex lens, the thickness T2 of the outer circumferential part of the lens is smaller than the thickness T1 of the center of the lens (T2<T1). Therefore, the opening area of the gate portion through which the molten resin flows into the cavity portion is smaller. A lens diameter D is substantially equal to a flow length L of the molten resin in the cavity portion. When the lens diameter D is greater than a lens thickness T (e.g., L/T<25), the viscosity of the molten resin which is filling the cavity portion rapidly increases, thus the resin pressure of the molten resin is not transmitted to the vicinity of the end (the side opposite to the gate portion) of the cavity portion, leading to insufficient filling with the molten resin. Thus, air collects in the vicinity of the end of the cavity portion, and parts that are not transferred to the injection molding die are generated, thus the shape accuracy may deteriorate.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 2006-272871, a depressed portion is provided along the circumferential part in the cavity portion as a measure to prevent the generation of weld lines in the concave lens, and the molten resin flowing along the circumferential part in the cavity portion flows into the depressed portion so that a protruding portion thus formed is disposed. Therefore, in the molten resins which have flowed into the cavity portion from the gate portion, the time difference is reduced between the arrival time of the molten resin flowing into the side opposite of the gate portion via the center portion of the lens and the arrival time of the molten resin flowing into the side opposite of the gate portion from the gate portion around the circumferential part. Consequently, the generation of weld lines in effective optical surface of the lens is prevented.